It is known that by distillation crude petroleum can be fractionated into a number of varied products which are distinguished from one another especially by their volatility.
In the feeding of spark ignition internal combustion engines the so-called petrol or gasoline fraction is used, which, as a result of its volatility, enables carburetting of the air to be effected before its introduction into the engine cylinders.
The Diesel cycle has enabled heavier fractions to be employed, such as gas-oil which is injected separately into the engine cylinders at the end of the phase of compression of the air and which ignites spontaneously due to the heat of compression.
Hitherto one has lived with this binary system of piston engine fuels and the corresponding two classes of engines.
For engines having carburettors where the air loaded with fuel is compressed in the cylinders before ignition it is advisable for the fuel to have anti-knock properties which have come to be characterized by the octane number.
The natural distillate from petroleum generally has a poor octant number which permits only an equally poor degree of compression. In order to improve the performance of carburettor engines one is compelled to increase this octane number by various artifices. One of them is the addition to the gasoline of products such as tetra-ethyl lead which unfortunately is very poisonous and hence presents a serious disadvantage in relation to pollution. In addition it contributes to the fouling of engines by deposits of lead and to corrosion of the valves.
Another artifice currently applied in petroleum refineries consists in taking fractions of the distillate which are respectively heavier and lighter than gasoline and in converting them by cracking or reforming operations in order to obtain hydrocarbons of aromatic or naphthene type having anti-knock properties, which are mixed with the gasoline fraction in order to increase its octane number.
But this procedure, apart from the fact that it necessitates complicated and costly apparatus, introduces losses resulting from the fuel that is burned for heating the apparatus and gives as residue gases which are generally burned in a flare. These losses may be evaluated at about 5% of the crude petroleum.
It is obvious that in the present economic situation these losses constitute a serious disadvantage.
On the other hand Diesel engines have made considerable technical progress and today for the propulsion of motor vehicles even in towns high-speed Diesel engines can be produced, having low weight, a certain flexibility in running and being not very noisy.